FIG. 5 is a circuit diagram of a double converter including a first converter which converts an alternating current voltage to a direct current voltage and a second converter which converts a direct current voltage to an alternating current voltage.
In FIG. 5, the two ends of a single phase alternating current power source 1 are connected to alternating current input terminals R1 and S1 via an EMI (Electro Magnetic Interference) filter 21. A ground capacitor 30 is connected between the alternating current input terminals R1 and S1 and a ground point, and a capacitor 33 is connected between the alternating current input terminals R1 and S1.
The alternating current input terminal R1 is connected to an A point, which is the connection point of semiconductor switching elements 50 and 51, via a reactor 60.
The series circuit of capacitors 31 and 32 and the series circuit of switching elements 52 and 53 are each connected in parallel to the series circuit of the switching elements 50 and 51, and the connection point of the capacitors 31 and 32 is connected to the alternating current input terminal S1. When the connection point of the switching elements 52 and 53 is taken to be a B point, a reactor 61 and a capacitor 34 are connected in series between the B point and the alternating current input terminal S1. Also, an EMI filter 22 is connected across the capacitor 34, and a load 7 is connected across the EMI filter 22. Furthermore, a ground capacitor 35 is connected between the two ends of the capacitor 34 and a ground point.
FIG. 5 shows a case in which an IGBT (Insulated Gate Bipolar Transistor) is used as each switching element 50 to 53, but there is also a case in which a MOSFET (Metal-Oxide-Silicon Field Effect Transistor) is used in place of an IGBT, or there is also a case in which a diode is used in place of the switching element 50.
In the double converter, an alternating current voltage is converted to a direct current voltage by an on/off operation of the switching elements 50 and 51 configuring the first converter, and the capacitors 31 and 32 are charged with the direct current voltage, while a direct current voltage is converted to an alternating current voltage by an on/off operation of the switching elements 52 and 53 configuring the second converter, and the alternating current voltage is supplied to the load 7.
Herein, the potentials at the A point and B point which are the connection points of the switching elements vary at a high frequency as a result of a switching operation of the switching elements 50 to 53.
As shown in FIG. 6, parasitic capacitances 40 and 41 exist between portions of the circuit, for example, the A point and B point and their respective ground points. Therefore, a high frequency leakage current I1 flowing via the parasitic capacitance 40 and ground capacitor 30, or a high frequency leakage current I3 flowing via the parasitic capacitance 41 and ground capacitor 35, circulates through the circuit as a result of potential variations at the A point and B point, thereby generating a disturbance voltage caused by common mode noise.
With this kind of power conversion device, there is a case in which an operation at a high switching frequency is required in order to achieve an increase in efficiency and a reduction in size, and there is also a case in which switching operates at a high frequency of, for example, several hundreds of [kHz]. Under this kind of high switching frequency, there is a tendency that as the time rate of change of potential (dv/dt) resulting from a switching operation increases, and the high frequency leakage currents increase, the disturbance voltage also increases eventually.
The EMI filters 21 and 22 function, as common mode noise filters for removing the noise generated by the switching operation, for external devices which share the alternating current power source 1. Consequently, by making the EMI filters 21 and 22 larger in capacity, it is possible to enhance a disturbance voltage reduction effect, but there is the problem of causing an increase in the size and cost of parts and the device.
A drastic measure for reducing the disturbance voltage is to reduce the size of the kinds of parasitic capacitances 40 and 41 shown in FIG. 6 to the minimum. Therefore, various ideas for, for example, the layout of parts and the route of wires have heretofore been devised, but all the ideas have limitations, and it is very difficult to completely remove the parasitic capacitance.
As an example of a heretofore known technology of reducing the disturbance voltage, the invention described in PTL 1 is known.
FIG. 7 is a circuit diagram showing the main portion of the invention described in PTL 1. In FIG. 7, Q is a semiconductor switching element, L is a reactor, N1 is a main winding, N2 is an auxiliary winding, C11 and C12 are capacitors, D1 is a diode, C1 is a parasitic capacitance existing between an X point and a chassis (ground point) of a device, C2 is a capacitor connected between the auxiliary winding and the chassis, Vin is an input voltage, and Vout is an output voltage.
As an operation of FIG. 7, when the potential at the X point varies as a result of an on/off operation of the switching element Q, and the leakage current I1 flows to the chassis via the parasitic capacitance C1, the current I3, which is equal in size and opposite in direction to the current I1, flows through the auxiliary winding N2 via the capacitor C2.
Herein, when the numbers of turns of the main winding N1 and auxiliary winding N2 are taken to be N1 and N2 which are the same as their respective reference signs, and the capacitance values of the parasitic capacitance C1 and capacitor C2 are taken to be C1 and C2 which are the same as their respective reference signs, by setting C1·N1=C2·N2, it is possible to cancel the leakage current I1 resulting from the potential variation at the X point, and thus possible to reduce the disturbance voltage.